This invention relates to a simple, low cost apparatus that can be easily installed underneath the keyboard of an acoustic piano for sensing musical note selection and note expression data, and a procedure for capturing the musical note selection and the note expression data.
There are many known ways of detecting a displacement and a displacement speed, or other note expression data of a keyboard for a musical instrument. The earliest known methods were mechanical switch structures. However, these had significant disadvantages, such as uniformity of response and unpredictable aftertouch control. Additionally, these types of switches affected key touch while playing, required time-consuming mounting, needed very tight vertical and horizontal adjustment, and generally had a high cost. For a further discussion of the problems of electro-mechanical switches see U.S. Pat. No. 4,628,786 issued to Buchla.
The apparatus, according to Buchla, overcomes some of the problems and disadvantages of these prior art mechanical switch structures by providing a pickup for each key that is moved within an electric field formed between a pair of stationary electrodes. Although still a mechanical system, since the pickup never contacts either of the electrodes, Buchla avoids many of the problems inherent in previous mechanical contact switches. A major advantage of the Buchla apparatus is that the voltage impressed on the pickup varies as a function of the position of the pickup within the electric field (i.e. voltage varies according to amount of key depression). The Buchla invention is thus a continuous linear position sensor providing the capability to capture greater note expression data, and provide for better velocity resolution. However, this apparatus is complex, costly, and requires time consuming installation and calibration. Additionally, the circuitry required to impress the AC voltages of equal amplitude but opposite phase on the two electrodes and the detection circuitry are again complex and costly.
In response to the above inherent limitations of mechanical switch structures, inventions utilizing various types of opto-electronic switches have been provided. One of the first was U.S. Pat. No. 4,351,221 issued to Starnes, et al. This system however requires two optical LED sensors per key and utilizes the sensors in a manner that creates a double contact system, thus eliminating the advantages of continuous linear position sensor. Additionally, this apparatus requires elaborate and delicate installation of photosensors. It is also expensive and requires the services of a skilled piano tuner or electronics technician.
Another opto-electronic switch mechanism is known from U.S. Pat. No. 4,362,934 issued to McLey. However, this apparatus has the same limitations as the system issued to Starnes, et al discussed above and further, is more commonly limited to electronic keyboards.
Another optical sensing means is known from U.S. Pat. No. 4,736,662 issued to Yamamoto. In order to reduce the number of displacement speed electrical signal converting elements and provide a design for use in a limited space, this apparatus utilizes optical sensor elements in conjunction with optical fibers. However, as described in the patent, the apparatus is complex and very costly. Additionally, the system would require delicate installation by a trained technician and fine calibration since the shutter system requires tight horizontal and vertical adjustments because the shutter affects the keying point of the key. Finally, it still operates as a two contact or two-step switch with the inherent disadvantages thereof compared to a continuous linear system.
Another recent invention is known from U.S. Pat. No. 4,768,412 issued to Sanderson. This system uses a single optical sensor per key and is designed for recording and scoring music and is by design portable and primarily applicable as a temporary add-on unit. It sits on top of the keyboard and is thus visibly obtrusive. Further, the patent itself indicates that it ". . . minimally interferes with the musician's movements as he plays . . . ". Additionally, the embodiment of the invention requires a piston and wiper assembly pair to be connected by adjustable connecting means to accommodate various key heights on different keyboards. Not only is this known plunger system with pistons, wipers and springs/channels complicated and costly, it is compounded given the fact that they are working in a linear range the length of the aperture of the photo-sensor. This requires very tight vertical resolution capability and adjustment problems. This invention also utilizes detection circuitry which compares the incoming analog voltage from the movement of the key with previously calibrated high and low voltage levels for purposes of determining key stroke velocity. However, this assumes that all keys have the same voltage differential for an equivalent vertical movement. Variations in key thickness and gain differentials in optical sensors will cause velocity resolutions to be very poor. This system also does not have the capability to provide actuation of the foot pedals of the piano to produce pedal signals.